1. Field of the Invention
The present invention relates to a laser light source, and more particularly to a laser light source from which light emitted from a semiconductor light-emitting device is emitted at a wavelength selected by an external resonator including a wavelength selector.
2. Description of the Related Art
A conventional laser light source is shown in Japanese Unexamined Patent Publication No. 10(1998)-254001 by way of example. The laser light source is constructed of a semiconductor light-emitting device (such as a semiconductor laser, etc.), and an external resonator having a wavelength selector (such as a narrow-band pass filter, etc.). The wavelength of a laser beam that is emitted is locked at a desired wavelength by operation of the external resonator. The external resonator can be constituted with an ordinary mirror, or a waveguide type wavelength selector in which, as disclosed in Japanese Patent Application No. 2000-19666, light is reflected at a wavelength selected by a reflection Bragg grating formed along an optical waveguide.
FIG. 17 shows a conventional laser light source equipped with the aforementioned waveguide type wavelength selector. The laser light source is constructed of a semiconductor laser chip 1, and a waveguide type wavelength selector 2 coupled directly to the front facet 1a of the semiconductor laser chip 1. The semiconductor laser chip 1 has a stripe 3, which includes an active region. The stripe 3 is formed perpendicular to both end facets (front facet 1a and rear facet 1b) of the semiconductor laser chip 1. The wavelength selector 2 consists of a channel type optical waveguide 4 formed in a ferroelectric substrate, and a distributed Bragg reflector (DBR) grating 5 formed in the optical waveguide 4 along the light propagation direction.
The front facet la of the semiconductor laser chip 1 is provided with an antireflection (AR) coating. Similarly, the end facets 2a, 2b of the wavelength selector 2 are provided with AR coatings, respectively. The rear facet 1b of the semiconductor laser chip 1 is provided with a high reflective (HR) coating.
In the aforementioned laser light source, light emitted from the semiconductor laser chip 1 is incident on the channel type optical waveguide 4 of the wavelength selector 2 and is reflected within the DBR grating 5. The DBR grating 5, and the rear facet 1b of the semiconductor laser 1 provided with a HR coating, constitute an external resonator. A laser beam 6 is emitted from the front facet 2b of the wavelength selector 2 at a wavelength that resonates in the resonator. Since the wavelength of the light reflected within the DBR grating 5 is selected according to the grating pitch, the emission wavelength is locked at a desired value in the emission gain of the semiconductor laser chip 1.
Japanese Unexamined Patent Publication No. 10(1998)-254001 discloses that an optical wavelength converter is coupled to a semiconductor light-emitting device and that a laser beam emitted from the light-emitting device is converted to a second harmonic by the optical wavelength converter. In this wavelength converter, for example, an optical waveguide extending in one direction is formed in a ferroelectric crystal substrate having a nonlinear optical effect, and domain-inverted portions inverting the direction of the spontaneous polarization of the substrate are periodically formed in the optical waveguide. In the optical waveguide, the fundamental wave traveling along the domain-inverted portions is converted to a second harmonic, etc.
In the aforementioned laser light source (where the external resonator equipped with the wavelength converter is combined with the semiconductor light-emitting device), even if the end facet, on the side of the wavelength selector, of the semiconductor light-emitting device is provided with an antireflection (AR) coating with a reflectance of about 0.1% which can be easily formed, the Fabry-Perot mode of the semiconductor light-emitting device tends to occur due to the residual reflection. Because of this, there is a problem that the Fabry-Perot mode will have an adverse effect on a longitudinal mode which is selected by the external resonator and will make the emission wavelength unstable.
More specifically, there are cases where the semiconductor light-emitting device oscillates at a wavelength differing from the wavelength of the feedback light from the wavelength selector to the semiconductor light-emitting device. In addition, there are cases where mode hops occur between the longitudinal mode of the external resonator and the longitudinal mode of a composite resonator (which is constituted with a Fabry-Perot resonator, constituted with both end faces of the semiconductor light-emitting device, and the external resonator) Furthermore, if the semiconductor light-emitting device is operated with high-frequency superposition in order to eliminate mode hops, it will result in disturbance. As a result, there are cases where the semiconductor light-emitting device oscillates at a wavelength differing from the wavelength of the feedback light from the wavelength selector to the semiconductor light-emitting device.
The aforementioned problems tend to occur particularly in the case where a high current is injected into the semiconductor light-emitting device to obtain high output, or the case where the longitudinal length of the semiconductor light-emitting device is relatively long and the gain has been increased.
Note that FIG. 18 schematically shows how the light output is fluctuated by the aforementioned mode hops. FIG. 19A schematically shows the emission spectrum of the laser light source when its light output is high; FIG. 19B schematically shows the emission spectrum of the laser light source when the light output is low.
To solve the aforementioned problems, a laser light source has been proposed which oscillates only at a wavelength selected by an external resonator. One such laser light source is disclosed in Jpn. J. Appl. Phys. Lett. 47(3), 1, Aug. 1985, pp. 183-185. In the laser light source, a semiconductor light-emitting device is coupled directly with an external resonator having a wavelength selector. The end facets that are coupled directly with each other are provided with coatings, which become AR coatings with respect to a wavelength selected by the external resonator. With this arrangement, a Fabry-Perot mode is not liable to occur between both end facets of the semiconductor light-emitting device, and consequently, a laser beam is liable to be emitted only by the external resonator. However, even if such a structure is adopted, it is fairly difficult to completely prevent the occurrence of the Fabry-Perot mode between both end faces of the semiconductor light-emitting device.